1. Field of the Invention:
The invention is related to air foils for wind energy conversion machines, and more specifically to air foils which limit the rotational speed of the machine to prevent overspeeding.
2. Description of the Prior Art:
It has long been known that wind energy can be harnessed and converted into mechanical rotational energy to create useful work. Typically, air foils are radially attached to a freely rotatable hub so that as air strikes the air foils a torque is created on the hub and the hub is rotated. The hub is usually connected to a drive shaft for either directly utilizing the mechanical power of the shaft for water pumping or grinding; or the shaft is connected to a generator for producing electrical energy.
A principal problem associated with wind energy is the damage associated with overspeeding of a wind machine when high winds produce high speeds of rotation. Overspeeding results when the speed of the machine exceeds it's design speed. If the overspeeding condition persists for very long the machine may be destroyed.
The most common method of reducing the force of the wind on the machine is to move a rudder vane, which normally positions the machine into the wind, parallel to the air foils so that the machine is pointed parallel to the wind. Such a configuration is disclosed in U.S. Pat. No. 2,376,020. Another means to lessen the force of the wind on the machine is to change the angular position of individual air foils to the wind. This maybe accomplished automatically by governors, auxiliary air foils, or other means that monitor the rotational speed of the air foils or the wind. Such wind machines that change the angular configuration of the the air foils to the wind in response the speed of the machine are disclosed in U.S. Pat. Nos. 2,106,557, 2,688,285, 4,105,363 and 4,111,601.
Changing the angular position of individual air foils necessitates the use of complicated linkages and assemblies to control the position of each air foil. An alternative to changing the angular position of each air foil, is to change the shape of each air foil. Changing the shape of each air foil can also limit the rotational speed of the machine and air foils preventing overspeeding. Flexible propellers for planes were proposed in U.S. Pat. No. 1,454,356, where a flexible propeller is disclosed which is flexible in both the longitudinal and transverse direction. In wind energy machines it has been proposed to twist the leading edge of the air foil to control the air foil's shape, as disclosed in U.S. Pat. No. 3,874,816. A more simplified procedure is to form the air foil of two or more pivotally connected pieces. Springs can then be used to hold the air foil in the proper configuration for normal wind conditions. During high wind conditions the spring tension is overcome and the air foil assumes another configuration with less surface area exposed to the wind. Air foils of this configuration are disclosed in U.S. Pat. Nos. 13,244, 192,668 and 1,403,069.
It is the object of the present invention to provide a simplified speed limiting means for an air foil, which is automatically actuated during high wind conditions.